1. Field of the Invention
The present invention relates generally to a variable damping force shock absorber for an automotive suspension system. More specifically, the invention relates to a variable damping force shock absorber which can provide substantially linear variation characteristics of damping force over various modes of vibration exerted on an automotive vehicle.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication No. 61-164836 discloses a variable damping characteristics shock absorber of the type to which the present invention is directed. In the disclosed construction, an orifice is formed through a piston for generating damping force in response to piston stroke according to relative displacement of a vehicular body and a suspension member which rotatably supports a road wheel. An end of the flow restriction orifice is closed by a disc valve which opens and closes the end of the orifice. A fluid passage is formed through a piston rod in a parallel relationship with the flow restriction orifice. A flow control means is associated with the fluid passage for adjusting the fluid flow path in the fluid passage for adjusting the damping characteristics.
In the shown construction, higher or harder suspension characteristics or a greater damping force may be generated by a greater magnitude of flow restriction provided by the flow control means. By a greater magnitude flow restriction, smaller amount of working fluid flows through the fluid passage for generating a greater fluid pressure difference at both sides of the piston thus generating a greater damping force. On the other hand, lower or softer damping characteristics are obtained by a smaller magnitude of flow restriction for allowing a greater amount of working fluid to flow through the fluid passage. A greater amount of fluid flow through the fluid passage may reduce the fluid pressure difference at both sides of the piston for generating a smaller magnitude of the damping force.
In such construction of shock absorbers, it has been observed that, at a relatively low piston stroke speed range, the flow control means of the fluid passage is principally effective for generating a damping force. On the other hand, at a relatively high piston stroke range, the orifice is principally effective for generating a damping force. Since the orifice and flow control means have different variation characteristics of magnitude of fluid flow restriction, a smooth variation of damping characteristics through a relatively wide piston stroke speed range is difficult to achieve.
Furthermore, neither of since neither the orifice nor the flow control means may provide linear characteristics in varying the damping characteristics, the prior proposed shock absorber is still not satisfactory to achieve both vehicular driving stability and riding comfort at any vehicular driving condition.
For improving the foregoing drawback in the prior art, U.S. patent application Ser. No. 332,469, filed on Apr. 3, 1989, now U.S. Pat. No. 4,905,799, issued on Mar. 6, 1990, which has been assigned to the common assignee of the present invention, discloses a shock absorber which provides piston stroke dependent variable shock absorbing characteristics by means of a first and a second flow restriction means associated with a piston. The first and second flow restriction means are arranged in a fluid path for communicating first and second working chambers defined within a cylindrical shock absorber housing in series. The first flow restriction means is associated with an orifice forming a part of the fluid path and has a predetermined relief pressure, for providing a predetermined first fixed flow restriction rate in response to a pressure difference between the first and second chambers smaller than or equal to the relief pressure and providing a second variable flow restriction rate in response to a pressure difference between the first and second chambers greater than that associated with the orifice for providing a third variable flow restriction rate variable depending upon the pressure difference between the first and second chambers.
The aforementioned prior proposal has been published as European Patent First Publication No. 03 36 692.
A further proposal has been made in the U.S. patent application Ser. No. 408,261, filed on Sep. 18, 1989, which has been assigned to the common assignee to the present invention, and in which is disclosed a shock absorber which provides a piston stroke speed dependent linear variation characteristics of damping force. The shock absorber includes variable orifices in tandem fashion for achieving linear variation characteristics of damping force according to variation of the piston stroke. One of the variable orifices provides a variation characteristics of flow restriction for a greater variation rate of the damping force in a low piston stroke speed range, and the other provides variation characteristics of flow restriction for a greater variation rate of the damping force in the intermediate and high piston stroke speed ranges. The variable orifice may be provided in a piston assembly or in the alternative in a bottom fitting in case of a double-action type shock absorber.
In either of the prior proposals, a fluid flow path for communication of the first and second flow restriction means or two variable orifices, extends through the piston in a substantial length. Because of the orifice effect of such fluid flow path and dimensional tolerance in formation of such fluid flow path, adjustment of the damping force to be generated by the shock absorber becomes difficult.